The Polyphonic Spree made a guest appearance with Micky and Peter during the encore of last night's show at the Moseley Folk Festival in Birmingham, England. The Polyphonic Spree have been covering "Porpoise Song" in concert for several years, and were also featured on the festival's bill, which took place over three days this past weekend. The Monkees headlined the festival's final evening. (I posted the set list from the show earlier.)
You can also see footage of the performance on The Monkees Tour Facebook page.